This invention relates digital data transmission systems and, more particularly, to a system for detecting data in a digital data transmission system in which data are transmitted by modulating the phase and amplitude of a carrier by discrete values at discrete times.
Digital data transmission systems are comprised of an emitter and a receiver interconnected by a transmission channel. In those systems the digital data, or bits, to be transmitted, appear as a sequence of bipolar binary pulses the frequency spectrum of which extends theoretically from zero to infinity. Mainly for saving purpose, telephone lines of the public network are used as transmission channels. Since telephone lines have generally a restricted pass band, 300-3000Hz, it is necessary to translate the energy of the bipolar binary pulses into the 300-3000Hz frequency band. Modulation, i.e., multiplication of the signal containing the information by a sinusoidal carrier, performs said translation. More particularly, this invention relates to the transmission systems that utilize modulation of the phase and amplitude of a carrier by discrete values at discrete times. These transmission systems include, in particular, the systems that utilize phase modulation, phase modulation combined with amplitude modulation, and quadrature-amplitude modulation, which modulation techniques are known as PSK (phase-shift-keyed) modulation A-PSK (Amplitude-phase-shift-keyed) modulation and QAM (quadrature-amplitude-modulation) modulation, respectively. PSK modulation is a modulation technique widely used and a detailed description thereof is provided, for example, in "Data Transmission" by W. R . Bennett and J. R. Davey, chapter 10, McGraw-Hill, New York, 1965, and "Principles of Data Communications" by R. W. Luckey, J. Salz and E. J. Weldon, Jr., Chapter 9, McGraw Hill, New York, 1968.
In the digital data transmission systems that utilize PSK modulation, the sequence of bits to be transmitted is, first of all, coverted into a sequence of symbols each of which can assume a discrete number of values, generally equal to a power of 2. Then, these symbols are transmitted, one by one, at times evenly separated by T seconds, and called sampling times, by varying the phase of the carrier in accordance with the value of these symbols, at sampling times. FIG. 1A shows a vector diagram illustrating the eight possible states of the carrier, at each sampling time, in a system using an 8 -phase PSK modulation for transmitting symbols capable of assuming eight distinct discrete values, each symbol being representative of a group of three bits. The amplitude of the carrier is constant and its phase can assume eight distinct discrete values.
In some digital data transmission systems, when the data transmission speed is to be increased and the number of the possible distinct discrete values of the phase of the carrier is not to be increased, A-PSK modulation is used. Briefly, in A-PSK modulation both amplitude and phase of the carrier are made to vary. For example, for a given sampling frequency F, if an 8-phase PSK modulation is used, a data transmission speed equal to 3F bits/sec. is obtained, since three bits are transmitted at each sampling time while using a two-amplitude level/eight-phase A-PSK modulation, a data transmission speed equal to 4F bits/sec. is obtained, since then four bits are transmitted at each sampling time. The vector diagram of FIG. 1B illustrates the 16 possible states of the carrier in a two-level/eight phase A-PSK modulation.
QAM modulation is a type of modulation technique which is more and more used, and a detailed decription thereof is provided, for example, in the above-indicated book by R. W. Lucky et al, Chapter 7, paragraphs 7-1-5 and 7-4-1. Briefly, in the digital data transmission systems using QAM modulation, the sequence of bits to be transmitted is, first of all, converted into the two sequences of independent symbols. Two symbols, one of from each of the two sequences, are simultaneously transmitted, at each sampling time, by varying the amplitude of two sub-carriers in quadrature, in accordance with the value of these symbols. These two subcarriers have the same frequency and their phases are shifted one with respect to the other, by .pi./2 radians. Then, the two sub-carriers are combined and applied to the input of the transmission channel. The vector diagram of FIG. 1C illustrates the 16 possible states of the carrier resulting from the combination of the sub-carriers, in a QAM modulation obtained by a four-level amplitude modulation of each of sub-carriers A and B.
The carrier modulated by one of the modulation techniques briefly described above, is applied to the input of the transmission channel. The function of the transmission channel consists of delivering at its output, a signal relatively similar to the one applied to its input. It was seen above that telephone lines are more often used as transmission channels. The telephone lines are well fitted for voice transmission but not for transmitting digital data at high speed, for example at 9600 bits/sec., with a low probability of error. These lines introduce distrubances which alter the quality of the signals during their transmission through those lines and render difficult a correct detection of the transmitted data by the receiver. These disturbances mainly include the amplitude and phase distortions due to the imperfection of the characteristics of the lines, and various noise components due, in particular, to the intermediate processing of the transmitted signals, performed by the public telephone network. The amplitude and phase distortions cause an interaction between the successively emitted signals, known as intersymbol interference. The noise components include in particular phase intercept, frequency shift, phase jitter and white noise.
The intersymbol interference and the noise components have practically no effect in the systems transmitting digital data at low speed, i.e., at speeds under 2400 bits/sec., but prohibit a correct detection of data in a system operating at high speed as indicated above. In the receiver of a high speed system, it is imperative to provide devices for minimizing the effects of the intersymbol interference and noise components, to obtain a correct detection of the data. The effects of the intersymbol interference are minimized by an equalizer not laying within the scope of this invention. The effects of the noise components are minimized by the detection system of this invention.
U.S. Pat. application Ser. No. 596,557 filed July 15, 1975, now U.S. Pat. No. 3,972,000 entitled , "Phase Filter For Reducing The Effects of the Noise Components Altering Discrete Phase Modulated Signals" and which is assigned to the assignee of the present invention, describes a phase filter minimizing the effects of the noise components affecting the phase of the carrier in a digital data transmission system. In general, in this phase filter, the noise components are cancelled by subtracting an estimated value of the noise components, from the phase value of the received signal. The phase value of the received signal minus the estimated value of the noise components, is applied to a decision logic which separates, the phase value of the emitted signal, representative of the data, and a residual noise component therefrom. Said residual noise component is applied to predictive filtering means which generate the estimated values of the noise components therefrom.
A first drawback of this phase filter lies in the fact that it allows the derivation of the phase value of the emitted carrier, representative of the data, from the phase value of the received carrier, and therefore, requires the use of a device to extract the phase value of the received carrier from said carrier. A second drawback of this phase filter is that it permits the detection of the correct phase of the carrier but does not provide any information about the carrier amplitude. The use of this phase filter in a system using A-PSK modulation or QAM modulation requires, therefore, in addition, the use of a device for detecting the correct amplitude of the carrier.
Therefore, the object of this invention is to over-come these drawbacks by providing an improvement to the phase filter described in the above-mentioned U.S. Patent application. This improvement is comprised of a system for detecting digital data, allowing a correct detection of data transmitted by modulating a carrier from the in-phase and quadrature components of the received signal.
Another object of this invention is to provide such a data detection system allowing a correct detection of the data transmitted by modulating the phase and amplitude of the carrier.
Generally stated, this invention provides a system for detecting digital data transmitted by modulating the phase and amplitude of the carrier, wherein the in-phase and quadrature components of the received signal, are applied to a device which causes the phase of the received sighal to rotate by an angle equal to an estimated value of the phase error produced by the noise components generated by the transmission channel. The new in-phase and quadrature components supplied by the phase rotation device, are applied to a decision logic which provides the detected phase and amplitude, representative of data, in accordance with reference coordinates and a given selection criterion. The decision logic provides, in addition, the components of the residual phase error which are applied to a first conversion device providing the value of the residual phase error from these components. The residual phase error is applied to a predictive filter supplying an estimated value of the phase error. This one is applied to a second device converting the estimated phase value into its sin and cos trigonometrical functions which control the phase rotation device. The components of the residual phase error can be used to adjust the equalizer of the data receiver including the detection system of the invention.